1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display device using the same.
2. Description of the Related Art
Liquid crystal display devices change optical anisotropy of a liquid crystal layer according to a voltage applied to the liquid crystal layer so as to change light transmittances thereof, and thereby displaying information such as character and image. Such liquid crystal display devices are mainly classified into three types: a transmission type; a reflection type; and a semi-transmission type, according to incident light for the display into the liquid crystal layer.
That is, in the liquid crystal display device of the transmission type, a backlight unit is disposed on a rear surface (non-display surface) side of a liquid crystal display element that is provided with the liquid crystal layer, and light from the backlight unit passes through the liquid crystal display element, whereby a user can visually recognize displayed information. Moreover, in the liquid crystal display device of the reflection type, incident light from a front surface is reflected by the liquid crystal display element, whereby a user can visually recognize displayed information.
Further, the liquid crystal display device of the semi-transmission type is designed to function similarly to the liquid crystal display device of the transmission type or the reflection type depending on an environment in which it is used. More specifically, the liquid crystal display device of the semi-transmission type displays by reflecting light from the outside in its environment in which the incident light from the outside is strong, similarly to the liquid crystal display device of the reflection type. Whereas, in an environment in which the incident light from the outside is weak, the backlight unit that is provided on the rear surface side of the liquid crystal display element is turned ON, so that the liquid crystal display device of the semi-transmission type display by using the light from the backlight unit, similarly to the liquid crystal display device of the transmission type. Further, some of the liquid crystal display devices of the semi-transmission type display in two modes including the transmission-type mode and the reflection-type mode at the same time, regardless of intensities of the incident light from the outside.
Moreover, a conventional liquid crystal display device, in which a backlight unit is provided with a prism sheet having a prism surface that is formed to have a sawtooth configuration, has been suggested (see, for example, JP 11(1999)-224058 A).
Here, the backlight unit provided in a first conventional example that is described in the above-described cited reference JP 11(1999)-224058 A will be explained specifically with reference to FIGS. 11A-11C.
As shown in FIG. 11A, a backlight unit 100 is provided with a planar light source device 101 that emits light with a flat shape (hereinafter, called “planar light”), and a prism sheet 104 and a reflecting plate 105 that are provided on an upper side and a lower side with respect to the planar light source device 101 in the figure, respectively, so that the planar light is incident into the liquid crystal display element (not illustrated) via the prism sheet 104.
The planar light source device 101 is provided with a light source 102 that emits light, a reflecting member 102a that is arranged so as to surround the light source 102, and a light guide member 103 having a wedge-shaped cross section that allows the light emitted by the light source 102 to be input therein and leads the input light toward a predetermined transmitting direction (illustrated as the arrow S1 in FIG. 11A). The reflecting member 102a reflects the light that is emitted by the light source 102 toward the light guide member 103, thereby allowing the light from the light source 102 to be incident into the light guide member 103 efficiently.
By also referring to FIG. 11B, the light guide member 103 is faces an output surface 103a that outputs the planar light toward the prism sheet 104 side, an inclined surface 103b that faces the reflecting plate 105 and is inclined by a predetermined inclination angle K1 with respect to the output surface 103a, and an input surface 103c from which the light from the light source 102 is input. In this light guide member 103, the light from the light source 102 that is input from the input surface 103c toward an inner side is repeatedly reflected by the output surface 103a and the inclined surface 103b or the reflecting plate 105 so as to be led toward the transmitting direction S1, and is output from the output surface 103a toward the prism sheet 104 appropriately.
In the prism sheet 104, a prism surface is arranged to face the output surface 103a of the light guide member 103 and is formed to have a sawtooth configuration. This prism surface is provided with a first prism inclined surface 104a and a second prism inclined surface 104b that are disposed alternately, and a ridge line 104c is formed on a boundary between these prism inclined surfaces 104a and 104b (see also FIG. 11C). Moreover, in the prism sheet 104, a vertical angle between the first and second prism inclined surfaces 104a and 104b is set as K2, as illustrated in FIG. 11C.
In the backlight unit 100 having the structure as described above, the light from the light source 102 is transmitted toward the transmitting direction S1 inside the light guide member 103. More specifically, when the light from the light source 102 that is input into the inside of the light guide member 103 is incident upon an interface between the output surface 103a and air at an incident angle that is smaller than a predetermined incident angle (total reflection angle), a portion of the light is refracted by the above-described interface and is output toward the prism sheet 104 side, and the remaining light is reflected by the interface toward the inner side of the light guide member 103, as shown by the arrows with the solid lines and the dotted lines in FIG. 11A. Moreover, the entire light incident upon the interface at an incident angle that is the total reflection angle or larger is reflected by the interface toward the inner side of the light guide member 103.
Moreover, the light that is output from the interface toward the prism sheet 104 side is incident upon the prism surface of the prism sheet 104, and is reflected by the second prism inclined surface 104b toward an upper side so as to be output from the prism sheet 104 as incident light into the liquid crystal display element, as shown by the arrows with the solid lines and the dotted lines in FIG. 11C.
Whereas, the light reflected by the interface between the output surface 103a and the air toward the inner side of the light guide member 103 travels toward the inclined surface 103b inside the light guide member 103, as shown by the arrow with the dotted line in FIG. 11A. Then, the light refracted by the interface between the inclined surface 103b and the air is output toward an outside of the light guide member 103, and is subsequently reflected by the reflecting plate 105 toward the light guide member 103 side. Further, this light passes through the inclined surface 103b so as to be incident into the light guide member 103 again, and is subsequently output from the output surface 103a toward the prism sheet 104 side so as to be output from the prism sheet 104 as the incident light, as shown by the arrows of the solid lines and the dotted lines in FIG. 11C.
As described above, in the backlight unit 100 in the first conventional example, the light that is output once from the inclined surface 103b of the light guide member 103 to the outside is reflected by the reflecting plate 105 toward the light guide member 103 side, so that the light output to the outside can be used for the display, thereby increasing an efficiency of utilizing the light from the light source 102. Moreover, in the backlight unit 100, by interposing the prism sheet 104, the light having high directivity with respect to the liquid crystal display element can be output.
Moreover, another conventional backlight unit, which is provided with a lens for gathering output light toward the liquid crystal display element at a position facing each of a plurality of prisms that are provided on a prism surface with a sawtooth configuration, is suggested (see, for example, JP 10(1998)-12024 A).
Here, the backlight unit of a second conventional example described in the above-described JP 10(1998)-12024 A will be described specifically with reference to FIGS. 12A-12C.
As shown in FIG. 12A, a backlight unit 200 is provided with a planar light source device 201 that emits planar light, and a prism sheet 204 that includes lenses for gathering light from the planar light source device 201 and outputting the light toward the outside, so that the backlight unit 200 allows the planar light to be incident into the liquid crystal display element, which is not illustrated, via the prism sheet 204.
The planar light source device 201 is provided with a light source 202 for emitting light, a reflecting member 202a that is arranged so as to surround the light source 202, and a light guide member 203 having a rectangular cross-section that allows the light emitted by the light source 202 to be input and leads the input light toward a predetermined transmitting direction (illustrated as the arrow S2 in FIG. 12A). The reflecting member 202a reflects the light emitted by the light source 202 toward the light guide member 203, thereby allowing the light from the light source 202 to be incident into the light guide member 203 efficiently.
By also referring to FIG. 12B, the light guide member 203 is provided with an output surface 203a that outputs the light from the light source 202 toward the prism sheet 204 side, a non-output surface 203b that is formed in parallel with this output surface 203a, and an input surface 203c from which the light from the light source 202 is input. On the output surface 203a, a prism surface having a sawtooth configuration is formed. That is, in the output surface 203a, a first inclined surface 203d that is formed to have a predetermined inclination angle K3 with respect to the output surface 203a, and a second inclined surface 203e that constitutes a prism having a isosceles triangular cross section with the first inclined surface 203d are provided, and a plurality of the prisms are provided along the transmitting direction S2. Then, in this light guide member 203, the light from the light source 202 that is input from the input surface 203c to the inside thereof is reflected repeatedly by the output surface 203a and the non-output surface 203b so as to be led toward the transmitting direction S2, and is output from the output surface 203a toward the prism sheet 204 appropriately.
The prism sheet 204 is disposed to face the output surface 203a of the light guide member 203, and is provided with a prism surface that is constituted of the plurality of the prisms so as to have a sawtooth configuration, and a plurality of lenses that are formed on an opposite side of the light guide member 203 of this prism surface and are disposed facing the liquid crystal display element. By also referring to FIG. 12C, each of the prisms of the prism surface is provided with a first prism inclined surface 204a and a second prism inclined surface 204b that are disposed alternately, and a ridge line 204c is formed on a boundary between these prism inclined surfaces 204a and 204b. Moreover, as shown in FIG. 12C, a vertical angle between the first and second prism inclined surfaces 204a and 204b is set as K5 in the prism sheet 204.
Moreover, in the prism sheet 204, a lens surface 204d having a semicircular cross section that protrudes toward the outer side is formed at a position facing the prism having the isosceles triangular cross section that is constituted of the prism inclined surfaces 204a and 204b. One end side and the other end side of each lens surface 204d are formed to be continuous to upper end sides of the prism inclined surfaces 204a and 204b, respectively, and a dimension of each lens surface 204d in the transmitting direction S2 is equal to a dimension of the prism in the transmitting direction S2. That is, in the prism sheet 204, the lens surface 204d that is included in each of the plurality of the lenses is provided for each of the plurality of the prisms that are arranged along the transmitting direction S2, thereby gathering the light reflected by the prism so as to output the light toward the liquid crystal display element side.
In the backlight unit 200 having the structure as described above, the light from the light source 202 is transmitted toward the transmitting direction S2 inside the light guide member 203. More specifically, when the light from the light source 202 that is input into the inside of the light guide member 203 is incident upon an interface between the output surface 203a and the air at an incident angle that is the total reflection angle or larger, which is shown as K4 in FIG. 12B, the light is reflected totally toward the inner side of the light guide member 203 as shown by the arrow with the solid line in FIG. 12B so as to travel in the transmitting direction S2.
Whereas, when the light is incident upon the second inclined surface 203e that is provided on the output surface 203a, the light is refracted by the inclined surface 203e so as to be output to the outside of the light guide member 203, as shown by the arrows with the dotted lines in FIG. 12B. Thereafter, the light output to the outside of the light guide member 203 is incident upon the prism sheet 204, is reflected by the second prism inclined surface 204b toward an upper side, and is subsequently refracted by the lens surface 204d in a substantially perpendicular direction so as to be incident into the liquid crystal display element, as shown by the arrows with the solid lines in FIG. 12A.
As described above, in the backlight unit 200 of the second conventional example, each of the plurality of the lens surfaces 204d provided on the prism sheet 204 gathers light, thereby enabling an increase in the directivity of the output light toward the liquid crystal display element.
However, the conventional backlight units as described above cannot increase the directivity of the output light, and may hardly increase brightness of the liquid crystal display device.
More specifically, in the backlight unit 100 of the first conventional example, it is necessary to increase the inclination angle of the inclined surface 103b shown as K1 in FIG. 11B so as to allow the light from the light source 102 that is incident into the light guide member 103 to travel upward along a normal line direction of the output surface 103a and to output the light from the output surface 103a efficiently. When the inclination angle K1 is increased as described above, an angle range of the light output from the output surface 103a to the outside (an angle difference between the arrow with the solid line and the arrow with the dotted line shown in FIG. 11B) is also increased. That is, an angle range of the incident angle of the light that is incident from the light guide member 103 upon the prism sheet 104 is also increased, an amount of the light that is output from the second prism inclined surface 104b toward the liquid crystal display element side at an angle inclined with respect to the normal line direction is increased, thereby decreasing the directivity of the output light from the prism sheet 104 (the backlight unit 100). As a result, an amount of the light that is incident into the liquid crystal display element in the perpendicular direction is decreased, and a half value of a brightness angle of the output light toward the liquid crystal display element is increased, thereby degrading the brightness (from a front view) of the liquid crystal display device.
Whereas, in the case of decreasing the inclination angle K1 of the inclined surface 103b in order to increase the directivity in the prism sheet 104, the light amount of the planar light that is output from the output surface 103a is decreased, that is, a plane output efficiency of the light on the output surface 103a is decreased. As a result, the efficiency of utilizing the light from the light source 102 is degraded, which consequently leads to the degradation of the brightness of the liquid crystal display device.
Moreover, in the backlight unit 200 of the second conventional example, the lens surface 204d, and the prism that is constituted of the first prism inclined surface 204a and the second prism inclined surface 204b are structured so as to have the same dimensions in the above-described transmitting direction S2. Thus, the second prism inclined surface 204b that mainly allows the light to travel from the prism side to the lens surface 204d is made smaller than the lens surface 204d, so that it is difficult to improve the light gathering efficiency on the lens surface 204d. Accordingly, it is also difficult to increase the directivity of the output light from the prism sheet 204 (backlight unit 200), so that it is not easy to increase the brightness of the liquid crystal display device. In particular, in a part near from the light source 202 of the prism sheet 204b, the light is likely to be incident upon the second prism inclined surface 204 at a large angle range similarly to the case of the backlight unit 100 of the first conventional example, and the lens surface 204d cannot gather the light in the normal line direction of the output surface 203a, so that the directivity of the output light from the backlight unit 200 may be decreased significantly.
In particular, in the case of applying the conventional backlight unit described above to the liquid crystal display device of the semi-transmission type, it is sometimes significantly difficult to increase the brightness of the liquid crystal display device. In detail, in the liquid crystal display device of the semi-transmission type, a plurality of reflecting electrodes are generally provided at a predetermined interval in a transmitting direction of light in a light guide member on an incident surface side of a liquid crystal display element. And, when utilizing the output light from the backlight unit, the liquid crystal display device of the semi-transmission type is necessary to allow the output light to pass through a transmission opening that is formed between the reflecting electrodes that are adjacent to each other in the transmitting direction, similarly to the liquid crystal display device of the transmission type. Thus, the liquid crystal display device of the semi-transmission type is required to allow the output light with high directivity to be incident from the backlight unit into the liquid crystal display element. However, in the above-described conventional backlight unit, the directivity of the output light is low, and an amount of the light passing through the transmission opening cannot be increased, so that it is very difficult to increase the brightness of the liquid crystal display device of the semi-transmission type.